The present invention relates to a protractor with an auxiliary ruler, and more particularly to a protractor having a clear auxiliary ruler detachably and rotatably connected thereto. The auxiliary ruler could be turned relative to the protractor to align an indicating line on the ruler with one of the angle marks on the protractor, so that children could read the degree of an angle being measured or to draw a line corresponding to the measured angle with the help of the auxiliary ruler.
FIG. 1 shows a conventional protractor 10 that is a transparent semi-circular plate provided with marks 101 to indicate 180 degrees of angles. To measure an angle, the protractor 10 is laid on a surface of an angle to be measured with one of the marks 101 aligned with an edge of the angle, so that a degree of the angle could be read from the protractor 10. The conventional protractor 10 of FIG. 1 is not suitable for use when the angle to be measured is on a rather small figure, because the marks 101 are provided along an arced edge of the protractor 10 and would locate beyond an area covering the angle to be measured. A user could only determine an approximate degree of the measured angle according to an invisible line extended from a closest mark 101 toward the center of the circle on the protractor 10. The degree of angle so determined is not perfectly accurate.
Moreover, the conventional protractor 10 of FIG. 1 is only suitable for measuring an angle shown on a plane drawing or on a plane surface of an article, as shown in FIG. 2, and could not be used to measure an angle on a three-dimensional object, such as a polygonal column, a pyramid, etc.
When the conventional protractor 10 is used to draw an angle on a plane, the draftsman needs to make a mark on the drawing to align with a mark 101 on the protractor 10 showing a desired degree of angle, and then draws a line connecting the mark and an origin of measurement to obtain a wanted angle. The conventional protractor 10 is therefore not so convenient in use.
It is therefore tried by the inventor to develop a protractor with an auxiliary ruler that eliminates the drawbacks existing in the conventional protractor and is particularly useful in helping children to quickly and easily measure, draw or read an angle and learn the meaning of angle.
A primary object of the present invention is to provide a protractor with an auxiliary ruler, wherein the protractor is a semi-circular plate provided at a center of circle with a through hole, so that a clear auxiliary ruler having a length close to a diameter of the protractor is detachably and rotatably connected to the protractor at the through hole. With an indicating line on the auxiliary ruler and degree marks on the protractor, the protractor of the present invention may help children to read an angle of an article being measured or to draw a line indicating an angle corresponding to a mark on the protractor, enabling children to easily understand the meaning of xe2x80x9canglexe2x80x9d and to learn to use angles.
Another object of the present invention is to provide a protractor with an auxiliary ruler detachably attached to the protractor, so that the auxiliary ruler could be removed from the protractor for use alone. The auxiliary ruler is provided with a plurality of equally spaced through holes. A user may use two pens along with the separated auxiliary ruler to draw circles and arcs. Children may be guided in such drawing activities to understand meanings of center of circle, radius, diameter, circumference, etc.